Displays have been used in computers, televisions and large billboards to generally provide information and thus have become essential for life. With the recent advancement of display technology and improvement in quality of life, there has been a demand for research and development of a display having multiple functions to improve the quality of life as well as a conventional function to simply provide information. Accordingly, in future-oriented research on flexible display and wearable display using thin and light materials, organic light-emitting diode (OLED) has presented lots of possibility and various indicators. An OLED display is a self-light emitting display device and can be manufactured into a lightweight and thin form. Further, the OLED display has an excellent contrast ratio, a wide viewing angle, and a high response speed. Furthermore, as compared to a liquid crystal display (LCD) which is also an important material for display, the OLED display does not require a backlight which occupies a large volume in the LCD and thus has many advantages in manufacturing of a flexible and transparent device.
An OLED is equipped with an organic compound layer formed between an anode and a cathode. The organic compound layer includes a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL), and an electron Injection layer (EIL). When a driving voltage is applied to the anode and the cathode, holes passing through the HTL and electrons passing through the ETL are combined into excitons in the EML, and the excitons emit visible light to implement the gradation of the organic light emitting display device while transitioning from an excited state to a ground state. Typically, a low molecular or high molecular organic light emitting material can be used as a material of the EML.
Meanwhile, if an organic light emitting device is manufactured using a material including light emitting quantum dots besides a low molecular or high molecular material, it is possible to manufacture a high-resolution organic light emitting device with excellent emission properties. Herein, the quantum dots refer to nano crystals of several nm in diameter and have optical, magnetic, and electrical properties different from those of bulk states. These properties vary depending on the diameter of a material. The quantum dots may have a dot, nanorod, or branched shape. If the quantum dots are formed into a dot shape, each quantum dot may include a core as a central part, an overcoating surrounding the core, and a cap molecule surrounding the overcoating and may have a size of from about 2 nm to about 20 nm. The core refers to a central part of the quantum dot and emits light. The overcoating surrounding the core covers the core and reduces non-radiative relaxation. The cap molecule suppresses agglomeration or precipitation of the quantum dots in a colloidal solution and enables the quantum dots to be stably dispersed. Further, if the quantum dots are formed into a dot shape, each quantum dot may include only a core or may include only a core as a central part and a shell as an overcoating surrounding the core.
However, the quantum dots are mainly formed of heavy metals such as cadmium (Cd) and thus may cause environmental pollution or the like.
The paper (Nanoscale, 2015, 7, 15873-15879) which is the background technology of the present disclosure discloses a method of preparing protein quantum dots by self-assembling proteins. However, this paper does not describe an organic light emitting device using protein quantum dots.